In recent years great strides have been made in mapping corneal contours. From the miniaturization of the Placido disc device, as taught in U.S. Pat. No. 4,772,115, to the derivation of quantitative refractive information from a two-dimensional video image of the cornea, as shown in U.S. Pat. No. 4,863,260, more accurate surgical procedures and better contact lens fitting have been made possible. In particular, the representation of corneal topography through the use of a color scale has given the ophthalmologist and the optometrist a better sense of the significance of the electronically obtained information.
In the most popular form of image acquisition apparatus, the corneal image is scanned in polar coordinates and different colors are displayed to show departures from the condition that would be exhibited by a perfectly spherical object. For example, when a perfectly spherical object is scanned, the circular disc of uniform color is displayed. When the video image of a cornea exhibiting the condition known as keratoconus is acquired by the apparatus of the aforementioned patent, the display shows a sequence of different colored and successively larger diameter regions radiating outwardly from the area of the conical apex. When a cornea exhibiting cylindrical astigmatism is scanned, two fan-shaped multi-colored areas positioned approximately 180 degrees apart but having their narrowest portions adjacent each other are displayed. Both "with the rule" and "against the rule" forms of astigmatism are readily distinguishable by the orientation of the fan-shaped colored areas.
While the color display has effectively been adopted as an industry standard and is in wide use, the color-encoded polar contour map provides only one representation of the surface contour. Because eye surgery is a particularly sensitive field requiring the exercise of considerable technical judgment and professional surgical skill that must be carried out under stressful conditions, the manner in which any information is presented to the ophthalmic surgeon must be carefully chosen. In the limited time available in the operating room the surgeon must acquire all of the data relating to the topography of the cornea, interpret that data and then exercise his professional judgment. The topographic information should be made available in a form best calculated to direct the surgeon's attention to significant surface feature. The color-coded polar may is one form of representation. Unfortunately, color coding by itself does not enable one to see the actual "hills and valleys" of the corneal surface.
In an article appearing at pp. 37-41 of the Asia-Pacific Journal of Ophthalmology of January 1991 by H. Hamano et al., a new form of displaying corneal topographic information is disclosed. In that article, the corneal topographic information contained in the polar plot, and advantageously provided by the apparatus disclosed in the aforementioned U.S. Pat. No. 4,863,260, is reprocessed to present the color topographic map in a "hills and valleys" form. This is accomplished by replotting into a Cartesian coordinate display the topographic information contained in a plurality of different circular traces taken over the corneal surface, the x-axis being used to present the polar position and the y-axis being used to plot the elevation of points. While the H. Hamano et al. article makes a useful contribution to the art of corneal information display, the direct conversion of the polar traces to Cartesian coordinates produces a welter of overlapping traces which provides no information as to which particular trace corresponds to a more apical path and which to a more limbal path.
It is an object of the present invention to provide a "hills and valleys" representation to the ophthalmic surgeon which more properly orients the traces and relates them to their radial position.